Stevens said the British investigation undertook "groundbreaking'' three-dimensional accident reconstruction and that "data from 186 million points" at the scene had been taken to create a simulation of the accident "within an accuracy of one centimeter.''
A BBC poll of 1,000 British adults found that 31 percent believed the crash was not an accident while 43 percent believed it was.
In a dispassionate summary of the 800-page report against a bland blue background at a London press conference, Stevens knocked down one conspiracy after another.
Stevens said that the investigation determined that Fayed had bought a $450,000 ring for Diana, but had not presented it to her.
"Certainly a ring was bought…but we don't know exactly when Dodi was intending on giving Diana that ring,'' he said
Stevens, a former Metropolitan Police commissioner, said investigators had "spoken to many of her family and closest friends and none of them" ever heard her say anything about marriage to Fayed.
"Her last conversations with friends and confidantes were to the contrary,'' he said. "Prince William also confirmed to me that his mother had not given him the slightest indication of plans'' to marry Fayed.
"She was not engaged and she was not about to get engaged,'' he said. "We believe that she never saw that ring,'' he said. "I don't know whether Dodi was going to ask her to marry him that night.''
In September, Diana's personal butler, Paul Burrell, discounted the pregnancy and engagement rumors in an interview with ABC News, and said Diana was secretly seeing another man.
"Dodi al Fayed only knew the princess for 26 days,'' he told ABC News' Kate Snow.
Burrell said Diana's true love at the time of her death was a British heart surgeon named Hasnat Khan, with whom she had shared a tumultuous two-year relationship.
"He didn't want to go public,'' Burrell said.
Stevens said the investigation confirmed that the mysterious white Fiat Uno that witnesses claimed to have seen speeding from the accident did not belong to photographer James Andanson, as investigators for Mohammed al Fayed have claimed.
"We are satisfied that James Andanson was at home with his wife" on the night of the crash "before flying to Corsica on assignment the following morning,'' Stevens said.
Andanson was found dead in a burnt out car in May 2000 in an apparent suicide. Mohammed al Fayed claimed he had been murdered. But, Stevens said, after speaking to Andandson's wife, friends and colleagues, the investigation confirmed the original reports that he did, in fact, kill himself.
The identity of the driver of the white Fiat Uno will likely never be answered, Stevens said. French investigation "couldn't locate the car and at this time it's very unlikely we would do so.''
Stevens suggested the possibility that the driver of the car may have been a useful witness, but may be reluctant to come forward out of fear of having violated France's "Good Samaritan" law. The law requires witnesses of an accident to stop and attempt to assist the victim.
Stevens went on to dismiss claims that Andanson was a secret agent, saying that "there is no evidence that James Andanson was an agent for any security agency.''
Addressing what some witnesses described as a "flashing light'' right before the accident, Stevens said, "We are confident that any theories concerning flashing lights …can be discarded as a cause for that crash.''